The present invention relates to an electromagnetic device which converts electrical energy into mechanical energy. Solenoid devices have long been known in which a movable armature element is moved between two positions in response to the application of electrical energy. In general, however, the speed of operation of such devices has been limited by the rather substantial mass of the armatures. Such an armature was required in devices of this type since the electromagnetic flux passed through the armature in a direction parallel to the direction of movement of the armature and it was necessary to provide substantial armature cross-sectional area in order to handle the substantial flux concentration in the armature without saturating.
Various bi-directional solenoids, such as shown in U.S. Pat. No. 2,989,666, issued June 20, 1961, to Brenner et al, have utilized a pair of stator coils which tend to pull an armature in opposite directions. Brenner et al discloses using a circuit for proportioning current to the stator coils such that the armature seeks a position in which the opposing forces applied thereto are balanced. U.S. Pat. No. 3,870,931, issued Mar. 11, 1979, discloses a bi-directional solenoid in which a pair of coils are energized in duty cycle fashion to drive an armature in the desired direction in dependence upon a command signal. Other prior art devices, such as shown in U.S. Pat. No. 2,274,775, issued Mar. 3, 1942, to Cox and U.S. Pat. No. 3,946,851, issued Mar. 30, 1976, to Cestrieres et al, disclose bi-directional solenoid arrangements in which the armature is moved to one of two stable armature positions in response to energization of the coils. By using more than two coils, a device, such as shown in U.S. Pat. No. 3,503,022, issued Mar. 24, 1970, to Burdett, can be constructed in which a discrete number of stable armature positions are attainable by energizing one or more of the solenoid coils.
In order to reduce substantially the mass of the armature of a bi-directional solenoid, thereby enhancing the speed of operation of the solenoid, a bi-directional solenoid operating on a reluctance principle was developed, as shown in U.S. Pat. No. 4,097,833, issued June 27, 1978, to Myers, assigned to the assignee of the present invention. Several of the embodiments disclosed in the Myers patent utilize annular air gaps defined in a pair of opposite-acting stators, with an annular armature arrangement being positioned such that it extends into both of the air gaps simultaneously. Each air gap is defined by a pair of concentric cylindrical pole surfaces with the annular armature overlapping each of the pole surfaces by areas dependent upon the position of the armature. It will be appreciated that with a relatively thin annular armature and narrow air gaps, the overlapping areas on the inner pole surfaces are substantially equal to the overlapping areas on the outer pole surfaces. When, however, the air gap is increased, the overlap areas of the inner pole surfaces will be appreciably less than the overlap areas of the outer pole surfaces. This is somewhat undesirable in a solenoid operating at substantial power levels, since the force generated by the solenoid will depend primarily on varying the smaller of the two overlappping areas, i.e., the inner pole surface overlap.
As shown in U.S. Pat. No. 3,900,822, issued Aug. 19, 1975, to Hardwick et al, assigned to the assignee of the present invention, bi-directional solenoids have taken advantage of the force varying characteristics of tapered pole pieces. The Hardwick et al solenoid, however, requires an armature of substantial mass, since magnetic flux passes through the armature parallel to the direction of movement of the armature.
Accordingly, a need exists for an electromagnetic device in which an annular armature cooperates with an annular air gap of a stator, but in which overlap areas between the inner and outer pole surfaces of the stator and the armature are sufficiently equal such that force is generated as a result of both overlap areas.